vBoys and girls are equal in number at conception, but more female fetuses die during pregnancy, leading to a slightly higher number of males being born, researchers said Monday.

The study in the Proceedings of the National Academy of Sciences is based on the largest dataset ever analyzed in the search to explain what is known as the human sex ratio, which has been poorly understood until now.

Documentation came from the United States, and included "the sex ratios of fetuses at different gestational ages, including three-to-six-day-old embryos produced by assisted reproductive technologies, induced abortions, chorionic villus sampling, amniocentesis, and United States census records of fetal deaths and live births," said the PNAS report, authored by researchers at Harvard University and Oxford University.

Scientists found that the human sex ratio is equal at conception, but that during gestation, there were certain times when male embryos were more likely to die than females, particularly in the first week after conception when there tended to be more abnormal male embryos than female embryos.

During the next 10 to 15 weeks, females faced a higher risk of mortality in the womb.

Later in pregnancy, male miscarriages were higher in number than female miscarriages, particularly in weeks 28 to 35, the study found.

But overall, more female died in the womb than males, the study showed.

"These are fundamental insights into early human development," the authors wrote.

Their findings contradict previous research that suggested more males are conceived than females, and that more males die during pregnancy than females.